A report’s found that fatigue cracking above the wheels close to the yaw damper bracket and anti-roll bar fixing points on Hitachi Class 800 trains was caused by the vehicles experiencing greater loads from train movement than allowed for in the original design. The Office of Rail and Road (ORR) says it’s not yet known for certain why this happened, although potential factors include wheel wear and track design.
The problem was discovered in last May last and resulted in severe disruption to GWR services in Devon. Stringent mitigation measures had to be put in place to allow the trains to re-enter service without passenger safety being compromised.
Additional cracks were found in the area where the lifting plates attach to the vehicle body. These were the result of stress corrosion cracking (SCC), resulting from the use of the particular type of aluminium in various areas of the Class 800 trains. ORR says characteristics of the specific aluminium alloy in combination with built-in stresses from being welded to the body and exposure to air containing chloride resulted in cracking.
Hitachi Rail and its industry partners are about to start the major SIX YEAR programme of work to repair 1,247 Class 800 series vehicles. It plans to replace the affected part of the original body structure, including the longitudinal welds where the fatigue cracks have occurred, and the mounting brackets. Hitachi Rail has also proposed to modify the addition of bolts to retain the plate in the unlikely event that the welds fail completely because of the stress corrosion cracking. The company hopes to minimise disruption to passengers while ensuring safety, with the minimum number of units out of service at any one time prioritising the trains for repair based on need, drawing on the monitoring data it has compiled from the fleet engineering and safety checks.
In its report published today, ORR is recommending that the rail industry should conduct further work to identify the reasons for the higher levels of fatigue loading experienced by rolling stock. Since the Hitachi Rail design complied with the applicable industry standards, evaluating whether the applicable standards consider the loads arising from operation on the rail network in Great Britain.
ORR says Hitachi Rail should also carry out a formal review of the effectiveness of their processes for welding when the component geometry is more challenging, which should include consideration of whether the existing approach adequately mitigates the risks of a weld with insufficient fusion being accepted. It wants designers of rolling stock to understand the risk posed by stress corrosion cracking and give it specific consideration when proposing the use of some aluminium components. ORR states that this does not mean that the specific type of aluminium should not be used in future, or that the aluminium used in the current trains compromises safety; but when used, steps should be taken to ensure protection of structures.
The rail industry as a whole should, ORR concludes consider whether a standard for mitigating stress corrosion cracking should be developed, as no dedicated standard currently exists and should develop a process for responding to similar future cross-industry crisis events and appoint a strong, independent chair who can maintain pace, focus, and ensure all voices are heard.
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